$\begingroup$Since the spectral class is really determined at birth from its mass, its spectral type has always been G2V - the path that it takes throughout is lifetime is similar to the paths taken by other G2V stars... I don't think that the spectral class system really applies to protostars.$\endgroup$
– MystaryPiOct 24 '18 at 22:03

$\begingroup$@MystaryPi Ok, interesting. So stellar classification is less an indication of surface temperature/luminosity, but rather determined strictly by mass?$\endgroup$
– Arkenstein XIIOct 24 '18 at 22:14

$\begingroup$Mass is one of the major factors that determine the spectral class of a star when they're born.$\endgroup$
– MystaryPiOct 24 '18 at 22:16

$\begingroup$@MystaryPi Thanks for your answer. Do you know whether it is possible to determine a star's Surface Temperature and Luminosity if you know its Mass and Age?$\endgroup$
– Arkenstein XIIOct 24 '18 at 22:20

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$\begingroup$Mass does not determine spectral type, since for almost all stars the mass is unknown (other than by appealing to relationships between mass and spectral type!)$\endgroup$
– Rob JeffriesOct 24 '18 at 22:42

3 Answers
3

Spectral type is determined by (and is an indicator of) photospheric temperature and surface gravity. The classification of G2V applies to a star with a photospheric temperature of about 5800K and a surface gravity of about 100-1000 m/s$^{2}$.

The Sun's temperature and gravity have not changed greatly since it first appeared on the "zero age main sequence" - it has got a little bigger and more luminous and a little bit hotter (by about 170 K- Schroder & Smith 2008) - so maybe it was a G3V-G4V star at the zero age main sequence (Mamajek 2018).

However, there were much bigger changes during the 20 million years or so before the Sun reached the ZAMS. It was cooler and briefly much larger. This would be reflected in a spectral classification that was something like K5IV- M0IV; indicating a subgiant-like gravity and a temperature of about 4000K. As it aged, it would then shrink and become hotter, passing through the G5V-K5V spectral types before reaching the main sequence.

If you can synthesise these two diagrams in your head, you can see what I mean.

It is true that on the main sequence, where most stars spend most of their lifetimes, mass is the main determining factor of all stellar parameters, with metallicity and angular momentum distant second-most important factors.

However when a star is born, from the moment it becomes detectable, it moves through several stages of structural evolution, which surely do not classify as the stellar class that it later has on the main sequence.

The evolutionary stages of this pre-main-sequence (PMS) star can be described in the astronomer's favourite diagram - the HR-diagram - via the Hayashi track.
The Hayashi track is for any star of given mass the sequence of colour-luminosity values that it attains during its PMS evolution.

As colour gives the stellar class, there is an evolution in stellar class for all stars, even on the main sequence.

However

since its birth, the Sun has brightened significantly.

is incorrect. Once the sun arrived at the main sequence, it still brightened, yes, but that means there was an increase in Luminosity. As the wiki page on the sun's evolution clarifies in the figure seen below, the temperature however didn't change much during that time, therefore also not the stellar class.

Summarizing
During the pre-main-sequence of a star the stellar class evolves significantly, on the main sequence it doesn't.

Mass is one of the major factors that determine the spectral class of a star when it is born.

During a star's protostar stages, a spectral class determines the length of its protostar stage and other factors such as its lifetime. Even though a spectral class is not physically assigned to them, the life stages that they undergo are essentially determined by their spectral class at birth.

So even though protostars don't have a spectral class determined by scientists, the star that it ends up becoming in its main sequence is really what its spectral class is.

So in this case, the Sun in it's protostar stages still was on the G2V route - it was taking the same (or at least similar) life cycles as stars in the G2V spectral class.

$\begingroup$Spectral type is measured from a spectrum. It is determined by temperature and to a lesser extent surface gravity. It is related to mass but not determined by it.$\endgroup$
– Rob JeffriesOct 24 '18 at 22:43

$\begingroup$Oh I didn't know that... I read from somewhere that the mass affects spectral class as well... Must have been incorrect... Looks like your answer seems to have it right - I will upvote it :)$\endgroup$
– MystaryPiOct 24 '18 at 23:39

$\begingroup$Two stars of different mass, but the same temperature and gravity would have the same spectral type. Other permutations of the same sentence would not.$\endgroup$
– Rob JeffriesOct 25 '18 at 6:34

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$\begingroup$Also note that the above link about the M-L relationship gives the oft-seen yet completely wrong explanation "Since higher mass means a larger gravitational force, higher mass must also mean that higher pressure is required to maintain equilibrium. If you increase the pressure inside a star, the temperature will also increase. So, the cores of massive stars have significantly higher temperatures than the cores of Sun-like stars." High mass MS stars have weak gravity and low pressure, so the explanation is wrong. M-L has little to do with nuclear fusion (stars have that L prior to the MS).$\endgroup$
– Ken GOct 28 '18 at 14:17